The present invention relates to an electric power steering apparatus for applying an assisting steering force to a steering mechanism of a vehicle by driving an electric motor according to the operation for steering the vehicle. More particularly, the present invention relates to a convergence control for returning a steering wheel to a neutral position in the electric power steering apparatus.
Conventionally, there is provided an electric power steering apparatus in which an assisting steering force is applied to a steering mechanism by driving an electric motor according to a steering torque given to a handle (steering wheel) by a driver. In this electric power steering apparatus, there is provided a torque sensor by which a steering torque given to the steering wheel (a steering unit) is detected. According to the steering torque detected by the torque sensor, an electric current command value is set as a target value of the electric current which is made to flow in the electric motor. According to the deviation between this electric current command value and the electric current value actually flowing in the electric motor, a voltage command value to be given to a drive unit of the electric motor is generated. For example, the drive unit for driving the electric motor includes: a PWM signal generation circuit for generating a pulse width modulation signal (PWM signal) of a duty ration according to the voltage command value; and a motor drive circuit composed of a power transistor which is turned on and off according to the duty ratio of the PWM signal. Voltage corresponding to the duty ratio, that is, voltage corresponding to the voltage command value is impressed upon the electric motor. An electric current flowing in the electric motor by this impression of voltage is detected by the electric current detector, and a difference between this detection value and the above electric current command value is used as deviation for generating the above voltage command value. As described above, in the electric power steering apparatus, feedback control is executed so that the target electric current (the electric current command value) which is set according to the steering torque can flow in the electric motor.
In the above electric power steering apparatus, convergence control is also executed for converging the steering wheel (a steering unit for steering the vehicle) to the neutral position. This convergence control is executed by correcting the target value of the motor current according to the steering speed and the running speed of the vehicle. In this case, the quantity of correction is referred to as “a convergence electric current value” or “a damping compensation electric current value”. A specific method of this convergence control is described as follows. For example, a damping compensation electric current map, which is a table for giving a relation between the damping compensation electric current and the steering speed, is previously prepared, and a vehicle speed gain map, which is a table for giving a relation between the gain to be multiplied by the damping compensation electric current value and the vehicle speed, is previously prepared. According to these maps, the steering speed at each point of time and the damping compensation electric current value according to the vehicle speed are determined.
As described above, in the conventional convergence control, the damping compensation electric current is set according to the steering speed and the vehicle speed. Therefore, even when no convergence control is required, the motor current is corrected according to the damping compensation electric current. For example, when the steering wheel is quickly rotated, that is, in the case of a quick steering operation, a driver feels steering operation of the steering wheel heavy due to convergence control. In other words, the correction of a motor current in the case of convergence control corresponds to the generation of torque in a direction opposite to the rotating direction of the steering wheel. Therefore, convergence control affects a driver's feeling of steering.